The invention relates generally to producing polycrystalline silicon, and particularly to producing multicrystalline silicon by the induction method for use in the manufacture of solar cells.
Crystal silicon is used for producing solar cells to converse solar energy into electrical energy. Single-crystal silicon is usually used for this purpose.
Recently completed research has demonstrated that polycrystalline silicon formed by the large crystals, so called multicrystalline silicon, allowed to reach the efficiency of converting solar energy into electric energy close to that of single-crystal silicon. Production capacity of equipment for producing multicrystalline silicon is several times higher than for single-crystal silicon, and its technology is easier than the technology for obtaining single-crystal silicon. The use of multicrystalline silicon enables to reduce the cost of solar panels and to start their production on an industrial level.
Currently in use is a process for producing multicrystalline silicon ingots by the induction method comprising a continuous supplying, and an induction melting of polycrystalline lump silicon batch material in a silicon melt-pool on a movable bed of a water-cooled crucible, casting the molten silicon to the shape of the melting space, and subsequent crystallizing a multicrystalline silicon ingot (U.S. Pat. No. 4,572,812). The melt-pool is contained within a skull formed using the water-cooled crucible consisting of the vertical copper tube sections cooled with water. The copper sections are separated by gaps and form a melting volume enclosed on the perimeter. Gaps between the sections permit inductor electromagnetic field to penetrate into the crucible melting volume. The melting volume can be shaped as a circle, a square, or a rectangle. During melting, a melt-pool fills up the entire transverse area of the crucible, thus resulting both in melting and casting of the silicon pool as an ingot of specific transverse size and shape. As the silicon batch melts, and the crucible movable bottom moves downwardly, the pool in the bottom part of the melt-pool is crystallizing. The speed of ingot movement corresponds to the speed of the lump batch melting in the upper part of the melt-pool. As a result of this known method, a long multicrystalline silicon ingot with a specified cross-section is produced and later on used in the production of solar cell plates.
A disadvantage of the above process for producing multicrystalline silicon ingots by the induction method is the appearance of thermal stress in the ingot resulting in lower quality degradation of plates produced using such ingot. Thermal stresses in the ingot and in the plates made of such ingot results in decreased efficiency of energy conversion by solar cells formed of these plates. In addition, the output factor of good plates also decreases due to their rupture caused by thermal stresses.
The identified problems are resolved by the process of producing multicrystalline silicon ingots by induction method that is described in EP 1254861. According to this known process there is provided additional heating of a silicon ingot obtained in the process of continuous casting by using heaters located underneath the water-cooled crucible, and additional heating of the ingot by plasma discharge of a plasmatron located above the water-cooled crucible. At the same time, the plasma discharge is scanning over the pool surface. Using this process enables controlled cooling of the obtained ingot within the predetermined temperature gradient at length. Electric circuit for the plasmatron is looped at the silicon ingot through a special contact that is arranged underneath the place where the ingot exits the processing chamber. The method provides reducing the temperature gradient over the silicon ingot radius down to 9 to 7° C./m, and thus results in achievement of a high efficiency (14.2 to 14.5%) of converting solar energy into electric energy for the plates made of this ingot.
However, during continuous melting and production of long ingots of multicrystalline silicon with a permanent supply of lump batch to the melt-pool, only at the beginning of the process, the concentration of impurities in the melt-pool is fitting with the concentration of impurities in the loaded batch. Concentration of impurities in an ingot is defined by the segregation factor of each impurity. Inasmuch as the segregation factor for typical impurities in the source material is lower than 1, the concentration of each impurity in the ingot is lower than its concentration in the melt. As much as the ingot grows longer, the concentration of impurities in the melt-pool increases due to its accumulation, and consequently the concentration in the produced multicrystalline ingot also increases. When the concentration of impurities in the pool exceeds its limit established for each specific impurity, the multicrystalline silicon becomes unsuitable for producing solar cells. The parts of the ingot with the concentration of impurities higher than the prescribed limit cannot be used for producing solar cells and are rejected, thereby significantly decreasing a fraction of produced solar cells with a high conversion efficiency.
The method of EP 1754806 comprises charging and start-up heating a lump of silicon charge material in a controlled atmosphere on a movable bottom within the melting space of a water-cooled crucible, creating a bath of molten silicon, and subsequently melting and casting the molten silicon to the shape of the melting space, crystallizing a multicrystalline silicon ingot, and controllably cooling the silicon ingot using a heating equipment set. As the multicrystalline silicon ingot cools down, it is removed from the processing chamber via a gas seal which prevents atmospheric air from penetrating into the chamber, and is cut down into cut-to-length sections by a cutter. In order to increase the efficiency of the method, after a permissible limit of impurities concentration is reached in the pool, the melting process is stopped, the melt-pool is crystallized, and a separation device is drawn down into the crystallized ingot in the melting volume of the water-cooled crucible to block the melting volume and prevent impure silicon from the bottom surface of the separation device from entering the upper surface. At the same time, the initial lump batch of silicon is supplied onto the upper surface of the separation device, and the operations are repeated starting from supply and start heating of silicon lump batch.
The prior art method has the following drawbacks.
When induction melting and casting are stopped, if impurities reach their critical content in the pool in the process of obtaining a long ingot (e.g., 14 m long), the entire upper part of the ingot—approximately 2.5 m long—located inside the heating equipment and processing chamber (above the gas seal) should run the step of controlled cooling. For this purpose the controlled cooling is carrying out in the mode similar to the one used for the whole ingot and this step takes approximately 30 hours. In addition, each step of inserting the separation device into the furnace melting volume and resuming the process of melting and casting takes about 7.2 hours. During this period, induction melting and casting of the pool is not provided.
The step of inserting a separation device into a melting volume requires high precision, since even a small misalignment error during the installation of the separation device may result in its jamming and damage of the water-cooled crucible, consequently resulting in the compulsory termination of melting and casting.
Also, inserting of a separation device made of foreign material—specifically silicon nitride, or graphite—into the melt-pool gives rise to contamination of the lower part of the ingot which becomes impure, and as a result to reduce quality and output factor of good silicon. The need to resume melting on the top of the already produced silicon ingot leads to the need for stopping its movement into the heating equipment and keeping it inside the water-cooled crucible for a long time. This results in uncontrolled cooling of this part of the ingot, appearance of thermal stresses and microcracks in that area, and consequently, in the need for rejecting the upper part of the ingot.